In a polar modulator, the amplitude modulation and phase modulation are processed separately before being combined to create the desired signal. The processing of the magnitude and phase modulations are different, which can introduce a relative time offset between the magnitude and phase signal components. If this relative time offset is not corrected, the desired signal will not be accurately generated, and distortion will result. Typically this distortion will result in an increase in out-of-band signal energy.
One approach to solving this problem is to process the signal with a sample rate that is fast enough so that the required time offset correction can be achieved with a simple digital register. This may be a reasonable approach if the signal bandwidth is low. However, for high bandwidth signals, such as CDMA and WCDMA, the relative magnitude/phase timing may need to be accurate to within one nanosecond or less. Such timing resolution would require a sample rate on the order of 1 GHz, which is not practical.
Another approach is to use analog delay elements. These delay elements would be placed in the magnitude and/or phase paths after D/A conversion. Unfortunately, this approach has several drawbacks, including: (1) the delay may vary with time, with temperature, and from part to part; (2) the delay provided by the analog delay elements may need to be calibrated, which is a drawback in a low-cost manufacturing environment; and (3) the analog delay elements may be difficult to integrate into an integrated circuit, which increases the number of required components.